The present invention relates to improvements in microprocessor controlled transmission systems of the type employing both mechanical shifting of couplers and powershift clutch control to effect gear speed selection. More particularly, the present invention relates to improvements in transmission systems to reduce the magnitude of speed changes during range shifting, reduce the possibility of sudden changes in speed, and reduce synchronizer wear.
Transmission having both mechanically shifted couplers and powershift clutches are well known in the art. These transmissions can be designed to provide a wide range of gear speed ratios between the input and output shafts and are well adapted for use in off-road vehicles such as agricultural tractors. The above-referenced applications disclose such a transmission having two range select levers which shift two synchronized couplers to select ranges of gears and two powershift control buttons for controlling solenoid actuated electro-hydraulic clutches to select one of four gears within the selected range.
In a transmission having both mechanically shifted couplers and powershift clutches, a sudden acceleration or deceleration occurs when a range select lever is shifted from one range to another. For example, if a range select lever selects the gear range 1-4 in one position and gear range 5-8 in a second position, shifting of the range select lever results in a change of four gears, i.e. gear 1 to gear 5 or gear 2 to gear 6, etc., because the selected powershift gear does not change. If the second range select lever controls gear selection between the ranges 1-8 and 9-16, a shift of this lever produces a change of eight gears and the acceleration or deceleration is even greater. Such accelerations/decelerations are highly undesirable because they have a tiring effect on the operator, result in more wear on the transmission, particularly the coupler synchronizers, and reduce the ability to precisely control the vehicle speed. In accordance with one aspect of the present invention, movement of a range shift lever controls powershift gear selection so that when the lever is moved from a low to a high range position the lowest powershift gear is selected and when the lever is moved from a high to a low range position the highest powershift gear is selected. This provides a smoother shift and reduces synchronizer wear.
In accordance with a second aspect of the invention, synchronizer wear is further reduced by placing at least one electro-hydraulic powershift clutch between the transmission input shaft and the synchronized couplers and, as a clutch pedal is depressed in preparation for a range shift, releasing the fluid from the hydraulic clutch before the clutch pedal is fully depressed. This disconnects the input shaft prior to the time the synchronized shift is initiated. Thus, a lower shift effort is required and viscous drag of the transmission gears in the wet sump begins slowing rotation of the gears prior to the time the synchronized shift takes place.